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A Planetary Health Perspective on Agroforestry in Sub-Saharan Africa

2019; Elsevier BV; Volume: 1; Issue: 3 Linguagem: Inglês

10.1016/j.oneear.2019.10.017

2590-3330

Todd S. Rosenstock, Ian K. Dawson, Ermias Aynekulu, Susan Chomba, Ann Degrande, Kimberly Fornace, Ramni Jamnadass, Anthony A. Kimaro, Roeland Kindt, Christine Lamanna, Maimbo Malesu, Kai Mausch, Stepha McMullin, Peninah Murage, Nictor Namoi, Mary Njenga, Isaac Nyoka, Ana Maria Paez Valencia, Phosiso Sola, Keith Shepherd, Peter Steward,

Urban Agriculture and Sustainability

Human activities change the structure and function of the environment with cascading impacts on human health, a concept known as "planetary health." Agroforestry—the management of trees with crops and livestock—alters microclimates, hydrology, biogeochemistry, and biodiversity. Besides the nutritional benefits of increased fruit consumption, however, the ways agroforestry affects human health are rarely articulated. This review makes that link. We analyze the pathways through which tree-based farm and landscape change affect food and nutrition security, the spread of infectious disease, the prevalence of non-communicable diseases, and human migration in Sub-Saharan Africa. The available evidence suggests that, despite some increased risks of infectious disease, agroforestry is likely to improve a diverse range of pressing health concerns. We therefore examine the factors determining agroforestry use and identify three drivers of social and environmental change that will determine the future uptake of agroforestry in the region. Human activities change the structure and function of the environment with cascading impacts on human health, a concept known as "planetary health." Agroforestry—the management of trees with crops and livestock—alters microclimates, hydrology, biogeochemistry, and biodiversity. Besides the nutritional benefits of increased fruit consumption, however, the ways agroforestry affects human health are rarely articulated. This review makes that link. We analyze the pathways through which tree-based farm and landscape change affect food and nutrition security, the spread of infectious disease, the prevalence of non-communicable diseases, and human migration in Sub-Saharan Africa. The available evidence suggests that, despite some increased risks of infectious disease, agroforestry is likely to improve a diverse range of pressing health concerns. We therefore examine the factors determining agroforestry use and identify three drivers of social and environmental change that will determine the future uptake of agroforestry in the region. Human activities have radically altered the environment. Carbon dioxide and other greenhouse gases emitted into the atmosphere from energy production, agriculture, transportation, and other sources increase ambient temperatures, shift the quantity and timing of precipitation, and affect the frequency of extreme weather events.1Intergovernmental Panel on Climate Change (2014). Climate change 2014: synthesis report. Contribution of Working Groups I, II and II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Core Writing Team, R.K. Pachaur, and L.A. 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Molyneux D. et al.Unhealthy landscapes: policy recommendations on land use change and infectious disease emergence.Environ. Health Perspect. 2004; 112: 1092-1098Crossref PubMed Google Scholar Clearing land and burning biomass contributes to widespread air pollution, morbidity, and mortality.12Koplitz S.N. Jacob D.J. Schwartz J. Myers S.S. Liu T. Pongsiri M. Buonocore J.J. Marlier M.E. DeFries R.S. Mickley L.J. et al.Public health impacts of the severe haze in Equatorial Asia in September-October 2015: demonstration of a new framework for informing fire management strategies to reduce downwind smoke exposure.Environ. Res. Lett. 2016; 11: 094023Crossref Scopus (106) Google Scholar Meanwhile, reactive nitrogen discharged into the air and water from fertilizers and fuel is associated with cancer, respiratory illnesses, and other health risks.13Townsend A.R. Howarth R.W. Bazzaz F.A. Booth M.S. Cleveland C.C. Collinge S.K. Dobson A.P. Epstein P.R. Holland E.A. Keeney D.R. Human health effects of a changing global nitrogen cycle.Front. Ecol. Environ. 2003; 1: 240-246Crossref Google Scholar These examples provide just a fraction of the emerging evidence of how human activities change the capacity of the planet's natural systems to support human health, a concept known as "planetary health."14Myers S.S. Lecture Planetary health: protecting human health on a rapidly changing planet.Lancet. 2017; 390: 1-9Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Humans have significantly modified the landscapes of Sub-Saharan Africa in efforts to improve welfare. This started in the savannahs, where grasslands were burned annually to minimize bush encroachment and improve hunting success.15Pausas J.G. Keeley J.E. A burning story: the role of fire in the history of life.Bioscience. 2009; 59: 593-601Crossref Scopus (435) Google Scholar Forests were then cut down for shelter, fuelwood, and access to fertile soils.16Ordway E.M. Asner G.P. Lambin E.F. Deforestation risk due to commodity crop expansion in sub-Saharan Africa.Environ. Res. Lett. 2017; 12https://doi.org/10.1088/1748-9326/aa6509Crossref Scopus (26) Google Scholar,17Cerutti P.O. Sola P. Chenevoy A. Iiyama M. Yila J. Zhou W. Djoudi H. Atyi R.E.A. Gautier D.J. Gumbo D. et al.The socioeconomic and environmental impacts of wood energy value chains in Sub-Saharan Africa: a systematic map protocol.Environ. Evid. 2015; 4: 1-7Crossref Scopus (11) Google Scholar Recently, the emergence of megacities has resulted in unprecedented flows of natural resources, including water and wood, from rural to urban areas.18Keys P.W. Wang-Erlandsson L. Gordon L.J. Megacity precipitation reveal teleconnected water security challenges.Glob. Environ. Chang. 2014; 27: 96-105Google Scholar Exploitation of the natural environment has supported dramatic population growth, as well as improvements in life expectancy and reductions in poverty for some. Even today, the majority of livelihoods in Sub-Saharan Africa are directly derived from natural resources, particularly from no- or low-input agriculture and pastoralism.19Alliance for a Green Revolution in Africa (2017). Africa Agriculture Status Report 2017: the business of smallholder agriculture in Sub-Saharan Africa.Google Scholar This continued direct reliance on natural resources makes Africans' health especially sensitive to environmental change, including climate change.20Samson J. Berteaux D. Mcgill B.J. Humphries M.M. Geographic disparities and moral hazards in the predicted impacts of climate change on human populations.Glob. Ecol. Biogeogr. 2011; 20: 532-544Crossref Scopus (50) Google Scholar However, that may also increase the efficacy and reduce the lag time of nature-based solutions to health challenges. Agroforestry—the integration and management of trees and woody shrubs with crops and livestock—is a frequently suggested solution to intertwined food-, climate-, energy-, land-, and water-related challenges in Sub-Saharan Africa. Seventy-one percent of African countries have committed to using agroforestry for climate change adaptation and/or mitigation in the Nationally Determined Contributions—blueprints for climate action—they submitted to the United Nations Framework Convention on Climate Change.21Rosenstock T.S. Wilkes A. Jallo C. Namoi N. Bulusu M. Suber M. Mboi D. Mulia R. Simelton E. Richards M. et al.Making trees count: measurement and reporting of agroforestry in UNFCCC national communications of non-Annex I countries.Agric. Ecosyst. Environ. 2019; 284: 106569Crossref Scopus (0) Google Scholar More than half the countries on the continent (28 out of 54) have pledged to ecologically restore a total of 1,130,000 km2 of land, and much of this restoration will rely on tree planting. Countries such as Kenya and Ethiopia have taken steps to conserve and restore landscapes with trees to preserve vital watershed functions. Development paradigms including "nature-based solutions," "climate-smart agriculture," "agroecology," "sustainable intensification," and "ecosystem-based adaptation" all promote agroforestry.22Food and Agriculture Organization of the United Nations (2013). Climate-Smart Agriculture Sourcebook.Google Scholar, 23Vignola R. Harvey C.A. Bautista-Solis P. Avelino J. Rapidel B. Donatti C. Martinez R. Ecosystem-based adaptation for smallholder farmers: definitions, opportunities and constraints.Agric. Ecosyst. Environ. 2015; 211: 126-132Crossref Scopus (40) Google Scholar, 24Pretty J. Intensification for redesigned and sustainable agricultural systems.Science. 2018; 362: eaav0294Crossref PubMed Scopus (10) Google Scholar, 25Altieri M.A. Nicholls C.I. Henao A. Lana M.A. Agroecology and the design of climate change-resilient farming systems.Agron. Sustain. Dev. 2015; 35: 869-890Crossref Scopus (137) Google Scholar With politics and practice aligning, agroforestry is likely to be a driver of environmental change in Sub-Saharan Africa in the future. Agroforestry is not only tomorrow's solution: trees already are widely scattered on farms and ranches, and in other managed landscapes. Zomer et al.26Zomer R. Trabucco A. Coe R. Place F. Trees on Farms: An Update and Reanalysis of Agroforestry's Global Extent and Socio-Ecological Characteristics. World Agroforestry Centre, 2014Google Scholar mapped the extent of trees on farms using satellite imagery and geo-datasets and found that nearly 30% of the agricultural land in Sub-Saharan Africa had at least 10% tree cover (registering at about this level in both 2000 and 2010), with nearly 40% of the population that lives in agricultural lands based in such areas (Table 1). It is therefore clear that agroforestry is a major current land use in Sub-Saharan African landscapes. In fact, the aforementioned may significantly underestimate the current extent of agroforestry on the subcontinent due to technical limitations in using satellite imagery to identify low-density tree cover common in agroforestry systems27Schnell S. Kleinn C. Stahl G. Monitoring trees outside forests: a review.Environ. Monit. Assess. 2015; 187: 600Crossref PubMed Scopus (16) Google Scholar and because agroforestry occurs in areas not officially defined as cropland.21Rosenstock T.S. Wilkes A. Jallo C. Namoi N. Bulusu M. Suber M. Mboi D. Mulia R. Simelton E. Richards M. et al.Making trees count: measurement and reporting of agroforestry in UNFCCC national communications of non-Annex I countries.Agric. Ecosyst. Environ. 2019; 284: 106569Crossref Scopus (0) Google Scholar For example, silvopastoral systems that integrate livestock and trees take place on grazing lands, and shade-grown commodity agroforestry systems (e.g., coffee and cocoa) often meet the formal definition of forests, and therefore may not be captured.28Bisseleua D.H.B. Missoup A.D. Vidal S. Biodiversity conservation, ecosystem functioning, and economic incentives under cocoa agroforestry intensification.Conserv. Biol. 2009; 23: 1176-1184Crossref PubMed Scopus (59) Google ScholarTable 1Areal Extent of Tree Cover on Agricultural Lands and Population Living in Landscapes with Tree Cover in Sub-Saharan AfricaTree Cover (%)20002010km2% of Total Agricultural LandPopulation (Millions)% of Persons Who Live in Agricultural Areaskm2% of Total Agricultural Land>101,089,27827.567.6371,137,86428.7>20528,60213.328.216582,06414.7>30345,3028.713.07353,9618.9Population estimates were not calculated for 2010. Source: Zomer et al.26Zomer R. Trabucco A. Coe R. Place F. Trees on Farms: An Update and Reanalysis of Agroforestry's Global Extent and Socio-Ecological Characteristics. World Agroforestry Centre, 2014Google Scholar Open table in a new tab Population estimates were not calculated for 2010. Source: Zomer et al.26Zomer R. Trabucco A. Coe R. Place F. Trees on Farms: An Update and Reanalysis of Agroforestry's Global Extent and Socio-Ecological Characteristics. World Agroforestry Centre, 2014Google Scholar Despite the pervasive presence of agroforestry in Sub-Saharan Africa, the diverse impacts of tree-based environmental change on human health are rarely articulated. This is even though it is widely known that agriculture has significant implications for the spread of infectious disease, the prevalence of non-communicable diseases, human nutrition, and migration of human populations. In this review, we target this space by examining how changes in the environment due to agroforestry may influence human health. We have considered relevant peer-reviewed literature, starting with the current authors' experience in the various subjects covered, and complemented by Google Scholar searches. Although not a formal systematic review, we have made efforts to include studies illustrating both positive and negative impacts of agroforestry on the environment and health to provide a balanced assessment. When available, we used recent meta-analyses to frame the discussion. In brief, we found that the evidence suggests that, despite some disease risks, agroforestry can positively affect human health outcomes across a broad range of concerns (Figure 1). Here, we first detail the evidence of the many links between agroforestry and health. Then, given the benefits of agroforestry and the current interest in promoting it, we discuss the social determinants of agroforestry use in Sub-Saharan Africa and three key drivers that will influence the capacity to improve planetary health with agroforestry going forward. Agroforestry is the purposeful integration and management of trees on farms and in wider landscapes, either through retaining existing trees, planting indigenous or exotic trees, or allowing trees to naturally regenerate. In Sub-Saharan Africa, land managers practice agroforestry in nearly all climatic zones and farming systems, from arid to humid and from extensive livestock to intensive crop-management systems (Figure 2). In all cases, trees modify the biophysical structure of land, directly affecting the environment at multiple scales, from processes occurring at the micron scale immediately beneath the tree (e.g., soil aggregation) to those at the global scale in the atmosphere (e.g., climate change). Knowledge of how agroforestry affects climate, hydrology, nutrient cycles, and biodiversity is a prerequisite to understanding agroforestry-mediated connections between environmental change and human health. Agroforestry regulates field-scale microclimate including air temperature, solar radiation, and wind speed. Changes result from the physiological habit of trees: the shade effect and the cooling effects of water vapor due to increased evapotranspiration. Change in temperature can be significant. Midday temperatures are reduced by up to 6°C under a canopy of Faidherbia albida (faidherbia) trees in Ethiopia compared with open fields,29Sida T.S. Baudron F. Kim H. Giller K.E. Climate-smart agroforestry: Faidherbia albida trees buffer wheat against climatic extremes in the Central Rift Valley of Ethiopia.Agric. For. Meteorol. 2018; 248: 339-347Crossref Scopus (14) Google Scholar and trees also reduce temperature at field level in Ghanaian cocoa agroforests.30Blaser W.J. Oppong J. Hart S.P. Landolt J. Yeboah E. Six J. Climate-smart sustainable agriculture in low-to-intermediate shade agroforests.Nat. Sustain. 2018; 1: 234-339Crossref Scopus (7) Google Scholar Furthermore, the presence of trees buffers temperature fluctuations, helping to maintain more optimal growing conditions for associated crops and livestock.31Van Noordwijk M. Bayala J. Lusiana B. Muthuri C. Agroforestry solutions for buffering climate variability and adapting to change.Clim. Chang. Impact Adapatation Agric. Syst. 2014; : 217-232Google Scholar Trees, however, do not always change microclimates in beneficial ways. Shade can reduce, sometimes significantly, the photosynthetic active radiation reaching crops. For example, in semi-arid areas of Burkina Faso, unpruned trees reduced photosynthetic active radiation by more than 50% compared with open fields, suppressing crop yield.32Bayala J. Teklehaimanot Z. Ouedraogo S.J. Millet production under pruned tree crowns in a parkland system in Burkina Faso.Agrofor. Syst. 2002; 54: 203-214Crossref Scopus (58) Google Scholar The impacts of agroforestry trees on climate at larger scales are positive and similar to those of forests; trees reduce temperatures and help mitigate climate change.33Bright R.M. Davin E. Halloran T.O. Pongratz J. Zhao K. Cescatti A. Local temperature response to land cover and management change driven by non-radiative processes.Nat. Clim. Chang. 2017; 7: 296-302Crossref Scopus (59) Google Scholar, 34Bastin J.-F. Finegold Y. 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Chang. 2017; 43: 51-61Crossref Scopus (143) Google Scholar Trees modify soil structure directly through root growth, reducing soil compaction and increasing water retention and infiltration. Trees reduce water evaporation from soil by lowering ambient temperatures, which also reduces transpiration of associated crops.38Bayala J. Sanou J. Teklehaimanot Z. Kalinganire A. Oudraogo S.J. Parklands for buffering climate risk and sustaining agricultural production in the Sahel of West Africa.Curr. Opin. Environ. Sustain. 2014; 6: 28-34Crossref Scopus (50) Google Scholar This is one of the reasons that farms in agroforestry systems often have greater moisture content than farms without trees.39Bayala J. Sanou J. Teklehaimanot Z. Ouedraogo S.J. Kalinganire A. Coe R. van Noordwijk M. Advances in knowledge of processes in soil-tree-crop interactions in parkland systems in the West African Sahel: a review.Agric. Ecosyst. Environ. 2015; 205: 25-35Crossref Scopus (28) Google Scholar Conversely, trees can compete with crops for water, depleting soil water close to them. Small-scale impacts of trees on soil-moisture dynamics are amplified with high tree density and fast-growing species. Under these situations, trees increase transpiration and can draw down water tables. When trees are absent, water runs off the land more quickly. In Ethiopia, for instance, agroforestry increased infiltration and reduced catchment runoff by up to 81%.40Nyssen J. Clymans W. Descheemaeker K. Poesen J. Vandecasteele I. Vanmaercke M. Zenebe A. Van Camp M. Haile M. Haregeweyn N. et al.Impact of soil and water conservation measures on catchment hydrological response—a case in north Ethiopia.Hydrol. Process. 2010; 24: 1880-1895Crossref Scopus (87) Google Scholar Thus, intermediate tree cover may be most appropriate for balancing water demand and increasing groundwater recharge.41Ilstedt U. Bargués Tobella A. Bazié H.R. Bayala J. Verbeeten E. Nyberg G. Sanou J. Benegas L. Murdiyarso D. Laudon H. et al.Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics.Sci. Rep. 2016; 6: 1-12Crossref PubMed Scopus (50) Google Scholar Trees (and forests) also link local to regional and global water cycles through recycling of rainfall. Precipitation, especially inland precipitation, is mediated by the hydraulic pump of evapotranspiration and tree volatile compounds.42Sheil D. Murdiyarso D. How forests attract rain: an examination of a new hypothesis.Bioscience. 2009; 59: 341-347Crossref Scopus (90) Google Scholar At least 40% of rainfall originates from evapotranspiration globally. This means that trees connect different locations, with land use in one location altering rainfall in another.37Ellison D. Morris C.E. Locatelli B. Sheil D. Cohen J. Murdiyarso D. Gutierrez V. van Noordwijk M. Creed I.F. Pokorny J. et al.Trees, forests and water: cool insights for a hot world.Glob. Environ. Chang. 2017; 43: 51-61Crossref Scopus (143) Google Scholar Trees influence the cycling of nutrients in agricultural fields, farms, and surrounding landscapes both directly and indirectly. They mine deep soil layers for nitrogen, phosphorus, and other critical nutrients, making these available to crops through the decomposition of their dropped biomass and root turnover.43Verchot L.V. van Noordwijk M. Kandji S.T. Tomich T.P. Ong C. Albrecht A. Mackensen J. Bantilan C. Anupama K.V. Palm C.A. Climate change: linking adaptation and mitigation through agroforestry.Mitig. Adapt. Strateg. Glob. Chang. 2007; 12: 901-918Crossref Scopus (255) Google Scholar Leguminous trees and shrubs form symbiotic relationships with bacteria to fix atmospheric nitrogen and accumulate it in soils and biomass.44Chikowo R. Mapfumo P. Nyamugafata P. Giller K.E. Mineral N dynamics, leaching and nitrous oxide losses under maize following two-year improved fallows on a sandy loam soil in Zimbabwe.Plant Soil. 2004; 259: 315-330Crossref Scopus (49) Google Scholar,45Baggs E.M. Chebii J. Ndufa J.K. A short-term investigation of trace gas emissions following tillage and no-tillage of agroforestry residues in western Kenya.Soil Tillage Res. 2006; 90: 69-76Crossref Scopus (65) Google Scholar This provides a natural fertilizer to crops and protein-rich fodder for animals.46Sileshi G. Akinnifesi F.K. Ajayi O.C. Place F. Meta-analysis of maize yield response to woody and herbaceous legumes in sub-Saharan Africa.Plant Soil. 2008; 307: 1-19Crossref Scopus (91) Google Scholar,47Korir D. Goopy J.P. Gachuiri C. Supplementation with Calliandra calothyrsus improves nitrogen retention in cattle fed low-protein diets.Anim. Prod. Sci. 2016; 56: 619-626Crossref Scopus (5) Google Scholar These processes all have the effect of increasing nutrient inputs and organic materials into agricultural systems, through litter, biomass, and root decomposition. Such inputs have been considered crucial in increasing land-use efficiency, particularly in Sub-Saharan Africa, where farmers often cannot source and/or afford commercial fertilizers, and base-nutrient levels in soils are often low. However, tree-based systems also present some challenges for nutrient management. Increased nutrient inputs do not always end up in the target crop or livestock and may leach into groundwater or be lost to the atmosphere.44Chikowo R. Mapfumo P. Nyamugafata P. Giller K.E. Mineral N dynamics, leaching and nitrous oxide losses under maize following two-year improved fallows on a sandy loam soil in Zimbabwe.Plant Soil. 2004; 259: 315-330Crossref Scopus (49) Google Scholar,48Rosenstock T. Tully K. Arias-Navarro C. Neufeldt H. Butterbach-Bahl K. Verchot L. Agroforestry with N2-fixing trees: sustainable development's friend or foe?.Curr. Opin. Environ. Sustain. 2014; 6: 15-21Crossref Scopus (0) Google Scholar Increases in soil moisture and carbon content, and concomitant increases in soil biodiversity, can increase rates of nutrient cycling; although generally positive, this also results in higher emission rates of carbon dioxide through soil respiration. Lastly, harvesting of tree products from farms, especially of young woody material from dense plantings, can export large amounts of nutrients, leading to nutrient depletion.49Shepherd K.D. Ohlsson E. Okalebo J.R. Ndufa J.K. Potential impact of agroforestry on soil nutrient balances at the farm scale in the East African Highlands.Fertil. Res. 1996; 44: 87-99Crossref Scopus (44) Google Scholar The addition of trees to agricultural landscapes results in diversification of microclimates both above and below ground, as well as diversified strata and, potentially, vegetation phenology. Thus, agroforestry can create new ecological niches for both beneficial and detrimental species. Generally, agroforestry systems support higher biodiversity of birds, insects, and small rodents compared with monoculture systems;50Clough Y. Barkmann J. Juhrbandt J. Kessler M. Wanger T.C. Anshary A. Buchori D. Cicuzza D. Darras K. Putra D.D. et al.Combining high biodiversity with high yields in tropical agroforests.Proc. Natl. Acad. Sci. U S A. 2011; 108: 8311-8316Crossref PubMed Scopus (208) Google Scholar, 51Bhagwat S.A. Willis K.J. Birks H.J.B. Whittaker R.J. Agroforestry: a refuge for tropical biodiversity?.Trends Ecol. Evol. 2008; 23: 261-267Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 52Henry M. Tittonell P. Manlay R.J. Bernoux M. Albrecht A. Vanlauwe B. Biodiversity, carbon stocks and sequestration potential in aboveground biomass in smallholder farming systems of western Kenya.Agric. Ecosyst. Environ. 2009; 129: 238-252Crossref Scopus (78) Google Scholar however, increases in biodiversity are not always associated with increases in functional diversity or conservation of rare and/or endemic species.53Harvey C.A. González Villalobos J.A. Agroforestry systems conserve species-rich but modified assemblages of tropical birds and bats.Biodivers. Conserv. 2007; 16: 2257-2292Crossref Scopus (0) Google Scholar Effects on biodiversity are borne out with microorganisms too, in that adding trees to systems increases the diversity and function of soil biota.54Pumariño L. Sileshi G.W. Gripenberg S. Kaartinen R. Barrios E. Muchane M.N. Midega C. Jonsson M. Effects of agroforestry on pest, disease and weed control: a meta-analysis.Basic Appl. Ecol. 2015; 16: 573-582Crossref Scopus (32) Google Scholar The effect of trees on associated biodiversity appears to be determined by tree species, arrangement, and management intensity. In summary, agroforestry affects the environment by intercepting sunlight, lowering ambient temperatures, reducing crop evapotranspiration, increasing water use in some cases, improving soil water-holding capacity and water infiltration, and enhancing carbon storage and biodiversity, among other mechanisms. A synthesis of studies conducted in Africa (mostly addressing field-scale effects) suggested that, in about 60% of cases where the agroforestry-environment relationship was investigated, trees improved the delivery of ecosystem services (Table 2). Although most of the studies showed largely positive impacts, the considerable number of studies that showed negative or non-significant effects of agroforestry suggests the possibility of diverse, often site